The invention relates to a device for applying a prestress to structural elements, in particular for the prestressing of blow moulding elements mounted in the area of mould carriers, with these elements being adjacent to each other in the area of a mould seam and each enclosing an internal space, which is designed for shaping a container manufactured by a blow process.
A typical operating cycle in connection with the production of containers made of thermoplastic material, in particular in connection with the production of containers made of a biaxially oriented polyethylene terephthalate, consists of using air forced into the containers to shape such containers, which are made, for example, out of an injection molded preform that has been suitably thermally conditioned. This creates pressures in the range of approximately 40 bars, which cause the blow mould elements surrounding the interior space to no longer be firmly attached to each other but to be slightly pushed apart despite the locking of the mould carriers. This causes the manufactured container to deviate from the intended shape. Specifically the volume of the container increases and the gap arising between the blow moulding elements appears as an elevation on the container.
The devices presently known for applying prestress are not sufficiently suitable to permit not only a cost-effective realization, but also a simple assembly and a long service life.
Therefore the purpose of this invention is to create a device of the above mentioned kind that through a simple mechanical design achieves high reliability and a long service life.
The invention solves the task by embedding in an elastomer at least locally a positioning element, which is encircled at a distance by a peripheral element, and by arranging between the peripheral element and the positioning element at least a portion of the elastomer, so as to form an elastic connection.
Through the connection of the positioning element with the peripheral element by an elastomer a restoring force is exerted due to the elastically deformed elastomer, following a deflection of the positioning element relative to the peripheral element, with this force leading to a return of the positioning element back into its original position. Additionally, the elastomer causes a seal, which makes it possible, for instance, to use a gaseous or liquid pressure medium to move the positioning element without additional substantial sealing measures. The distance between the positioning element and the peripheral element is arranged so as to ensure that a sufficient volume of elastomer is present, which may be sufficiently deformed elastically to effect a multiplicity of deflection movements.
According to one preferred embodiment of the invention the front and back sides of the positioning element and the peripheral element, which are connected by an intermediate space located between the peripheral and positioning element, are each provided with an elastomer layer, which enter into the elastic connection arranged between the peripheral and positioning element. This layer increases the sealing qualities and provides an additional supply of elastomer, which is available for elastic deformation. Additionally, the contact between the elastomer and the position and peripheral elements is improved thereby, decreasing the danger of detachment.
According to another preferred embodiment of the invention vulcanized rubber is used as an elastomer. Vulcanized rubber has a high degree of permanent elasticity and attaches very well to surfaces covered by it, if it is vulcanized directly to the surfaces of the peripheral element and the positioning element.
According to another preferred embodiment the peripheral element is stretched around the positioning element in an enclosing fashion and the positioning element essentially is in the form of plates. The closed version of the peripheral element maintains its shape during the application of pressure to change the location of the positioning element. Furthermore, a reliable guide is created for the positioning element and a mechanically stable mounting is created, for instance in the area of a mould carrier. The platelike design of the positioning element provides a large adhesion area. This makes it possible to impose pressure over a large area to the blow moulding elements as well as provide a large working surface for the pressures effecting a change in location. In particular, it is intended to make available a small filling volume in order to allow a fast pressure build-up. The pressure build-up in the filling volume takes place more rapidly than the pressure build-up in the container.
Further details of the present invention follow from the detailed description below and the attached drawings, which graphically show the preferred embodiments of the invention.